1. Field of the Invention
This invention relates to secondary electrochemical cells and more particularly to secondary electrochemical cells utilizing a molten sodium negative reactant, a solid ionically-conductive separator, a molten sulfur and molten aluminum halide positive reactant, and a molten sodium haloaluminate electrolyte.
2. Description of the Prior Art
It may be explained here that U.S. Pat. No. 3,404,035 discloses a secondary battery that in its fully charged condition has a molten sodium negative reactant, a molten sulfur positive reactant and a solid electrolyte-separator of beta alumina or its derivative disposed between the positive and negative reactants. In its discharged or partially discharged condition, it has an additional electrolyte comprising a sodium polysulfide (Na.sub.2 S.sub.x) on the positive reactant side of the beta alumina separator.
The potential of this battery is typically in the range of from about 2 to 2.2 volts and the operating temperature of this battery is in the range of 265.degree. to 350.degree.C. in order to keep the discharge products, Na.sub.2 S.sub.x, molten at all times. It is recognized by those skilled in the art, however, that operation of this type battery at about or above 265.degree.C cannot be carried out without attendant problems. For example, Na.sub.2 S.sub.x is a strong oxidizing agent at these elevated temperatures and will tend to effect rapid corrosion and chemical attack of containers, current collectors, seals, and separators of the battery, thus seriously reducing the useful life of the battery.
A novel secondary battery has now been discovered which utilizes a molten sodium negative reactant, a molten sulfur and aluminum halide positive reactant in contact with a molten sodium haloaluminate electrolyte. This battery can operate at a temperature as low as from about 150.degree.C. to about 225.degree.C. and thus greatly reduce the corrosion and chemical attack problems associated with sodium-sulfur batteries of the type described above. Additionally and surprisingly, it has been found that the potential of this novel battery approaches or exceeds 2.66 volts.